Electrode negative pulse welding system and method

ABSTRACT

A welding system includes a power source configured to generate power and deliver the power to a welding torch. The power is provided in accordance with an electrode negative pulse welding regime that includes a cyclic peak, followed by a stabilization phase, then a return to a background level. The stabilization phase has a generally parabolic current shape, and is performed in a current-closed loop manner until a transition point, where control becomes voltage-closed loop until the background level is reached. Resulting weld performance is improved, with a globular-like transfer mode, reduced shorts and enhanced arc stability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of Ser. No. 13/828,040, filed Mar.14, 2013, entitled “Electrode Negative Pulse Welding System and Method”,in the name of Bryan Dustin Marschke et al., which is hereinincorporated by reference.

BACKGROUND

The invention relates generally to welding processes, and morespecifically, to methods and systems for controlling electrode transferin pulsed spray gas metal arc welding (GMAW-P) processes.

Welding is a process that has become ubiquitous in various industries,and may be used to facilitate many metal construction and assemblyapplications. For example, one process commonly known as gas metal arcwelding (GMAW) is most generally a specific welding process that uses awelding arc between a continuous filler metal electrode and a workpiece.Certain GMAW derivation processes or transfer modes such as spraytransfer and pulsed spray transfer (e.g., GMAW-P) may include relativelyhigh voltage levels, high amperage levels, and high wire feed speed(WFS) to transfer droplets of the metal electrode material across thewelding arc onto relatively thin metals workpieces. Unfortunately, whenusing an electrode negative polarity welding arc, the metal electrodemay be reluctant to transfer material across the welding arc.

Thus, while it would be advantageous in many applications to utilize apulsed electrode negative welding regime, conventional techniques wouldadd too much energy to the weld, create bridging shorts and inconsistentmetal transfer, erratic arc length, and may result in unwanted spatter.Improvements in the field that would permit such waveforms to beutilized while improving welding performance would be an advance in theart.

BRIEF DESCRIPTION

In one embodiment, a welding system includes [to be completed followinginitial review].

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an exemplary GMAW system in accordance withthe present disclosure;

FIG. 2 is an exemplary elevational view of the welding electrode of theGMAW system of FIG. 1 using a direct current electrode negative (DCEN)polarity in an improved pulse welding process;

FIG. 3 is an exemplary elevational view of the electrode shown in FIG. 2illustrating material transfer during a peak phase of the pulse weldingprocess;

FIG. 4 is an exemplary timing diagram of the pulse welding processvoltage and amperage waveforms; and

FIG. 5 is a detailed graphical representation of certain phases of thepulsed welding process.

DETAILED DESCRIPTION

Welding processes have become ubiquitous in various industries, and maybe used to facilitate metal construction and assembly applications. GMAWis most generally a specific welding process that uses a welding arcbetween a continuous filler metal electrode and a workpiece. CertainGMAW derived processes or transfer modes such as spray transfer andpulsed spray transfer (GMAW-P) may include relatively high voltagelevels, high amperage levels, and high wire feed speed (WFS) to transferdroplets of the metal electrode material across the welding arc toperform welding tasks on relatively thin metals workpieces.Unfortunately, when using an electrode negative polarity welding arc,the metal electrode may be reluctant to transfer material across thewelding arc.

Accordingly, present embodiments relate to systems and methods useful inadjusting one or more characteristics of voltage and amperage outputlevels to improve transfer metal electrode across a DCEN pulsing weldingarc, as well as arc stability. Specifically, reducing the falling edgetransition of each peak pulse in a pulse welding regime creates a“stabilization phase” between the peak and a background phase, allowingsufficient time and slow responsiveness at a current-closed loop outputfor deposition to settle while avoiding or reducing the change for a“hard short” that requires clearing. In prior techniques, aggressivecurrent control following the peak phase tended to cause rapid voltagechanges, arc instability, spatter, and frequent short circuits. Othercharacteristics of the voltage and amperage output levels such as pulsefrequency, background period, and pulse width may also be adjusted toimprove arc control. As used herein, “stabilization phase” may refer tocontrol of current (and voltage) following a peak phase of a pulsewelding regime, prior to transition to a phase in which voltages (andcurrents) return to a background level. The stabilization phase willtypically be used and with DC electrode negative pulse weldingtechniques, and may characterized by a parabolic, current-closed loopdecline in welding power output. The stabilization phase may beterminated at a higher programmed current than in conventional pulsewelding regimes. Then, in a “return to background” phase following thestabilization phase, a proportional-only gain is used for thevoltage-closed loop control. It should be appreciated, however, that thetechniques described herein may not be limited to spray transfer andpulsed spray transfer GMAW processes, but may also be extended to otherGMAW processes. Indeed, as discussed below, rather than a spray-typetransfer, the stabilization phase tends to promote a more globulartransfer of filler metal to the weld puddle, particularly when used withan EN polarity.

With the foregoing in mind, it may be useful to describe an embodimentof an welding system, such as an exemplary GMAW system 10 illustrated inFIG. 1. The system illustrated may be typical for an automated orsemi-automated (e.g., robotic) welding system, although the arrangementillustrated may be altered in many ways, and the techniques may also beused in hand-held welding processes. As illustrated, the welding system10 may include a welding power source 12, a welding wire feeder 14, agas supply system 16, and a welding torch 18. The welding power source12 may generally supply welding power for the welding system 10. Forexample, the power source 12 may couple to the welding wire feeder 14via a power cable 20, as well as to via a lead cable 22 to a workpiece24, such as through a clamp 26. In the illustrated embodiment, thewelding wire feeder 14 is coupled to the welding torch 18 via a weldcable 28 in order to supply, for example, a metal cored weldingelectrode and power to the welding torch 18 during operation of thewelding system 10. In some arrangements, the wire feeder may beincorporated into the power source. Gas from the gas supply system 16 isalso typically routed through the weld cable 28. Regarding theworkpiece, it is believed that the present techniques may beparticularly well suited to workpieces comprising relatively thin gaugegalvanized (or coated) steels, although other materials and sizes ofmaterials may be welded as disclosed. Moreover, various travel speedsmay be accommodated by manual, or more typically robotic movement of thetorch, the workpiece, or both, such as travel speeds of at least 30in/min, although other speeds may be utilized as well.

The welding power source 12 may further generally include powerconversion circuitry (not separately shown) that receives input powerfrom a power source 30 (e.g., an AC power grid, an engine/generator set,or a combination thereof), conditions the input power, and provides DCor AC output power for welding. The welding power source 12 will alsoinclude output terminals for providing welding power output, and thesemay allow for connection in accordance with either positive or negativepolarity welding regimes. Specifically, the welding power source 12 maypower the welding wire feeder 14, and by extension, the welding torch 18in accordance with demands of the welding system 10. In certainembodiments contemplated by this disclosure, the welding torch 18 may becoupled to the power supply and wire feeder to implement an EN weldingregime, and in particular, a pulse welding process. That is, the powersource 12 may be useful in providing a DCEN output, in which theelectrical current flows through the completed circuit from the negativeto positive direction, and thus affects the welding arc and/or weldingprocess. In addition to a DCEN output, the power source 12 may alsoinclude circuit elements (e.g., transformers, rectifiers, switches, andso forth) capable of converting the AC input power to a direct currentelectrode positive (DCEP) output, DC variable polarity, pulsed DC, or avariable balance (e.g., balanced or unbalanced) AC output to perform oneor more welding processes.

For GMAW embodiments, the welding system 10 also includes the gas supplysystem 16 to supply a shielding gas or shielding gas mixtures from oneor more shielding gas sources to the welding torch 18. The shielding gasmay be any gas or mixture of gases that may be provided to the weldingarc and/or weld pool in order to provide a particular local atmosphere(e.g., to shield the welding arc, improve arc stability, limit theformation of metal oxides, improve wetting of the metal surfaces, alterthe chemistry of the weld deposit, and so forth). For example, theshielding gas may comprise one or a mixture of argon (Ar), helium (He),carbon dioxide (C_(O2)), oxygen (_(O2)), and nitrogen (_(N2)).

Accordingly, as previously noted, the welding torch 18 generallyreceives the metal welding electrode from the welding wire feeder 14,and a shielding gas flow from the gas supply system 16 in order toperform a welding operation on the workpiece 24. During operation, thewelding torch 18 may be brought near the workpiece 22, such that thewelding electrode 32 approaches the workpiece and a welding arc 34 isestablished. It is further believed that the present techniques may beparticularly useful with particular types of electrode wires. Forexample, the electrode 34 may be a metal cored welding wire suitable foruse with a DCEN welding polarity. In such cases, the electrode willinclude a sheath consisting of metal encircling one or more metal cores.The welding electrode may also include fluxing or alloying componentsthat may act as arc stabilizers and, further, may become at leastpartially incorporated into the weld. One metal cored welding wireuseful for DCEN pulse welding in accordance with the present techniquesis disclosed in U.S. patent application Ser. No. 13/743,178, entitledSystems and Methods for Welding Electrodes, filed on Jan. 16, 2013, byBarhorst et al., which is hereby incorporated into the presentdisclosure by reference.

In certain embodiments, the welding power source 12, the welding wirefeeder 14, and the gas supply system 16 may each be controlled andcommanded by a control circuitry 36. The control circuitry 36 willinclude one or more processors 38 and cooperating data processing andsensing circuitry that may be communicatively coupled to a memory 40 toexecute instructions stored in the memory for carrying out the presentlydisclosed techniques. These instructions may be encoded in programs orcode stored in tangible non-transitory computer-readable medium, such asthe memory 40 and/or other storage. The pulse welding techniques willtypically be pre-programmed for specific wire types and sizes, and theparticular process desired may be selected by a welding operator via aninterface (not separately shown). The processor 38 may be a generalpurpose processor, system-on-chip (SoC) device, application-specificintegrated circuit (ASIC), or other processor configuration. Theprocessor 38 may also support an operating system capable of supportingapplications such as, for example, Pro-Pulse™, Accu-Pulse™, Accu-Curve™,and Profile Pulse™ available from Illinois Tool Works, Inc. Similarly,the memory 40 may include, for example, random-access memory (RAM),read-only memory (ROM), flash memory (e.g., NAND), and so forth. As willbe further appreciated, in one embodiment, the memory 40 of the controlcircuitry 36 may be flash updated (e.g., via wired and/or wireless datatransmission, programming, and so forth) to include instructions to varyone or more parameter characteristics of the welding output power, andby extension, the welding arc 34. It should be noted that in manyconfigurations, separate processing and control circuitry may beprovided for the power supply and for the wire feeder. The power supplytypically performs the processing of the control signals used to controlpower electronic devices (e.g., SCRs, IGBTs, etc.) for producing desiredoutput. In presently contemplated embodiments, code defining the DCENpulse welding process utilizing a stabilization phase is stored in thememory 40 and executed by processing circuitry in the power supply.

As noted above, components of the control circuitry 36 iscommunicatively coupled to (or embedded within) the welding power source12, the welding wire feeder 14, and gas supply system 16, and, as notedprovides control of one or more parameters (e.g., voltage and amperageoutput, wire feed speed, travel speed for automated applications, etc.)associated with each of the aforementioned components.

FIG. 2 depicts an embodiment of a welding process using a DCEN polarityelectrical welding arc 34. As previously noted, the welding electrode32, once energized and positioned near the workpiece establishes anelectrical welding arc 34 to perform a weld of the workpiece 22.Specifically, when using a DCEN polarity welding arc 34, heating willparticularly take place in the wire electrode, resulting in lesspenetration than with DCEP processes. In such processes, the electrodeis designated as “negative”, while the workpiece is “positive”. Electronflow, indicated by arrows 42 is from the electrode 32 to the workpiece,and primarily to the weld puddle 44. Such techniques are sometimesreferred to as “straight polarity”. In general, an arc length 46 ismaintained between the tip of the electrode and the weld puddle 44. Thisarc length may be determined to some degree, and in many respectscontrolled by the power input to the electrode, and therethrough to thearc, the weld puddle, and the workpiece. While in many prior arttechniques efforts are made to rigorously control the arc length, thepresent technique, through use of the stabilization phase followingpulse peaks, tends to emphasize arc stability over strict control of thearc length.

Moreover, in conventional GMAW-P processes, transfer of metal from theelectrode tends to be in a spray mode. In these techniques, the weldingpower supply pulses the welding output with high peak currents set atlevels that create spray transfer, and low background current levelsthat maintain the arc, but that are too low for any metal transfer tooccur. Because the metal transfer during the background phase of thecycle, the weld puddle may freeze slightly.

While the present technique may be classified generally as a GMAW-Pprocess, it tends to differ from conventional processes in severalimportant respects. For example, conventional GMAW-P processes controlthe decline in current levels from the peak based on a linearrelationship between current and time (e.g., A/ms). They also tend toclose control loops (on current and/or voltage) to more rigorouslymaintain arc length, and transition to a voltage phase at a currentlevel lower then in the present technique. Moreover, such existingtechniques typically use a proportional/integral gain for voltage-closedloop control on the return to background portion of the ramp followingthe pulse peak. A consequence of these factors is that voltage andcurrents decline aggressively, which can result in frequent shortcircuits that may require clearing before the subsequent peak.

The present technique, particularly when used with EN polarities,generates a “softer” down ramp, emphasizing arc stability and avoidingor reducing the risk of short circuits. Moreover, as illustrated in FIG.4, the transfer mode tends to be more globular then conventional GMAW-Pprocesses. While transfer occurs during the peak phase, materialcontinues to be melted from the electrode thereafter, and one or moreglobules 48 tends to remain near or somewhat suspended between theelectrode and the weld puddle. The arc length 46 may change, or may bedifficult to rigidly qualify, although short circuits, and particularly“hard shorts” are typically avoided and the arc tends to be more stable.

Here again, while the waveform may be used with electrode positivepolarities, it is believed to be particularly useful when welding withelectrode negative polarities and processes. For control, the powersupply control circuitry may regulate the power output by cyclicallytransitioning between voltage-closed loop control and current-closedloop control. During the time the welding power output is low (e.g.,during a background phase of the pulsed waveform), the welding arcremains established, but will add little energy to the electrode andworkpiece, although heating of the electrode and weld puddle willcontinue. During this background phase, the electrode and pool areallowed to cool somewhat, and between the peak phase and the backgroundphase a stabilization phase is implemented as discussed more fullybelow. Again, the majority of metal transferred from the electrode willbe transferred during the peak phase of each pulse. This stabilizationphase that follows each peak phase reduces weld puddle instability andspatter, reduces the energy input to the weld (at least in part byavoiding “hard shorts”, mitigates porosity, and reduces “burn through”of the workpiece.

FIG. 4 shows an exemplary DCEN pulsed welding process 50 illustrated interms of an exemplary voltage trace 52 and current trace 54 over severalsequential cycles of pulsed welding. During each cycle, a voltage ramp56 is the leading edge of a voltage peak 58, followed by a stabilizationphase down ramp 60, which is current-closed loop, and a voltage-closedloop ramp 62 back to a background voltage level 64. Corresponding phasesmay be seen in the current waveform 54. That is, a current-closed loopramp 66 is implemented rising to the voltage-closed loop controlled peak68. During the peak, the controller may vary the current to maintain thevoltage at the desired level. In practice, a desired voltage command isissued during the peak phase, although the actual voltage may vary basedon the dynamics of the arc, occasional shorts that may occur, and soforth. Thereafter, a current-closed loop, generally parabolicstabilization phase ramp 72 drives the current down to a transition to avoltage-closed loop ramp 74 to return to the background level 78. Thesame cycle is then repeated throughout the welding operation.

By way of example, in one embodiment, the rising edge portion 66 of thecurrent waveform 54 may be controlled at a ramp rate of approximately600 A/ms. Upon achieving peak amperage 68, the control circuitry willmaintain a desired voltage peak, such as approximately 200 V during apeak period 70. The generally parabolic stabilization phase 72 ofcurrent-closed loop control will then be implemented during a time 76until the current has reached a programmed transition point. Here, andthroughout the present disclosure, it should be borne in mind that theparticular voltages, currents, ramp rates, and so forth will typicallybe programmed (“trained”) in advance, optimized for particular wires andwire sizes, and so forth. Moreover, in some systems, some degree ofoperator or programmer control of the parameters may be provided.

FIG. 5 illustrates the peak, stabilization, and return phases of thecurrent waveform in somewhat greater detail. As shown, the current peak80 begins at a background level 78. At transition point 82, then, alinear ramp 66 is initiated, such as at a rate of between 450 and 650A/ms to a peak current transition point 84, such as between 210 and 400A. Of course these ranges are exemplary only, and will typically bedifferent for different wire sizes and wire feed speeds. In a presentlycontemplated embodiment, the transition at this point may actually occurbased on one of two considerations. That is, the current may reach aprogrammed level, as mentioned, or the voltage may reach a programmedpeak value before the current reaches that level, resulting in atransition before the current limit is reached. Thereafter, during thepeak phase, the current “floats” to maintain the voltage at a desiredlevel in a voltage-closed loop manner. Following the period for thispeak, as indicated by transition point 86, the stabilization phasebegins that includes a decline in the current through current-closedloop control.

The generally parabolic shape of the current waveform during thestabilization phase results from implementation of acurrent-per-unit-time-squared (i/t2) relationship during the rampeddecline in current. Once the current reaches a transition point 88, suchas between 25 and 325 A, control again transitions to voltage-closedloop control, and the current waveform will exhibit a shape resultingfrom the control attempting to maintain the desired voltage decline tothe background level. It should be noted, however, that the transitionpoint for exiting the stabilization phase may vary for different wiresizes and ratings, and may be programmable within one or more ranges.For example, for 0.045″ wires, the exit point may be programmed between100 and 325 A; for 0.040″ wire it may be programmed between 50 and 275A; and for 0.035″ wire it may be programmed between 25 and 225 A. Theprogrammed value tends to be roughly 25 to 50 A higher thancurrent-control-to-voltage-control transitions in peak down ramps inexisting pulse welding regimes (and where the current begins to regulateduring the return to background levels under voltage-closed loopcontrol). Moreover, in a currently contemplated implementation, the gainapplied during this “return” phase of voltage-closed loop control is, ina presently contemplated embodiment, proportional only (although othergain relationships may be used). It is believed that the combination ofthe parabolic stabilization phase, the earlier exit point, and the useof a proportional-only gain for the return to background levels,separately and/or together, produce better control of arc stability(prioritized over arc length), and result in less frequent shorts, andthe tendency to avoid “hard shorts”.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A welding system, comprising: a power source configured to generatewelding power and deliver the welding power to a welding torch, whereinthe welding torch is coupled to a negative output terminal of the powersource; a welding wire feeder configured to advance a metal coredelectrode into the welding torch at a rate of advancement; and controlcircuitry configured to implement an electrode negative pulse weldingregime comprising a closed loop peak phase, a generally parabolic closedloop stabilization phase following the peak phase, and a closed loopreturn phase following the stabilization phase.
 2. The welding system ofclaim 1, wherein the stabilization phase comprises a down ramp ofcurrent defined by a current-per-unit-time-squared relationship.
 3. Thewelding system of claim 1, wherein the return phase comprisesvoltage-closed loop control with a proportional only gain on voltage. 4.The welding system of claim 1, wherein a leading edge of the peak phasecomprises a linear current-closed loop ramp to a pre-determinedtransition point.
 5. The welding system of claim 1, wherein the peakphase comprises a voltage-closed loop phase with a voltage command ofthe welding power during the peak phase is between 18 and 28 v.
 6. Thewelding system of claim 1, wherein a transition between thestabilization phase and the return phase is programmable between 25 and325 A.
 7. The welding system of claim 6, wherein the transition betweenthe stabilization phase and the return phase is above 50 A.
 8. Thewelding system of claim 8, wherein the transition between thestabilization phase and the return phase is above 100 A.
 9. The weldingsystem of claim 1, wherein the pulse welding regime produces a generallyglobular transfer of molten metal from the electrode to a weld puddle.10. A welding method, comprising: creating a linear close loopcontrolled ramp to a desired peak transition; closed loop regulatingwelding power during a peak phase; creating a non-linear closed loopramp during a stabilization phase to a desired return transition; andcreating a closed loop return to a background power level; wherein thesteps are performed cyclically throughout a welding operation with anelectrode negative polarity.
 11. The method of claim 10, wherein thewherein the stabilization phase comprises a down ramp of current definedby a current-per-unit-time-squared relationship.
 12. The method of claim10, wherein the return phase comprises voltage closed-loop control witha proportional only gain on voltage.
 13. The method of claim 10, whereinthe method is performed with a metal cored welding wire electrode. 14.The method of claim 10, wherein the peak phase comprises avoltage-closed loop phase with a voltage command of the welding powerduring the peak phase is between 18 and 28 v.
 15. The method of claim10, wherein the a transition between the stabilization phase and thereturn phase is programmable between 25 and 325 A.
 16. The method ofclaim 10, wherein the pulse welding regime produces a generally globulartransfer of molten metal from the electrode to a weld puddle.
 17. Anon-transitory computer-readable medium having computer executable codestored thereon, the code comprising instructions for: creating a linearclose loop controlled ramp to a desired peak transition; closed loopregulating welding power during a peak phase; creating a non-linearclosed loop ramp during a stabilization phase to a desired returntransition; and creating a closed loop return to a background powerlevel; wherein the steps are performed cyclically throughout a weldingoperation with an electrode negative polarity.
 18. The non-transitorycomputer-readable medium of claim 17, wherein the wherein thestabilization phase comprises a down ramp of current defined by acurrent-per-unit-time-squared relationship.
 19. The non-transitorycomputer-readable medium of claim 17, wherein the return phase comprisesvoltage-closed loop control with a proportional only gain on voltage.20. The non-transitory computer-readable medium of claim 17, wherein themethod is performed with a metal cored welding wire electrode.
 21. Thenon-transitory computer-readable medium of claim 17, wherein the peakphase comprises a voltage-closed loop phase with a voltage command ofthe welding power during the peak phase is between 18 and 28 v.
 22. Thenon-transitory computer-readable medium of claim 17, wherein the atransition between the stabilization phase and the return phase isprogrammable between 25 and 325 A.